Soft printing paper

ABSTRACT

Printing paper superior in a texture, a feel and ease in turning the page is provided by adjusting the product of the paper density, the breaking length of a paper-making direction and the Young&#39;s modulus of the paper-making direction of the paper made by a paper machine within the limits of no less than 2×10 18  and no more than 10×10 18  g·N/m 4  by using the following means independently or in appropriate combination: a means of lowering paper density by increasing the compounding ratios of low-density pulp and low-density filler, using bulking chemicals or reducing press pressure during paper-making process, a means of lowering the paper breaking length of a paper-making direction by improving the compounding ratios of filler, and a means of lowering the Young&#39;s modulus of the paper by using a softening agent.

TECHNICAL FIELD

[0001] The present invention relates to printing paper that is superiorin suppleness and is bulky, and particularly relates to printing paperpreferred for books.

BACKGROUND ART

[0002] Qualities such as a texture, a feel and ease in turning the pageare important for bookpaper. Particularly recently, paper qualities suchas lightness in spite of heftiness (i.e., tall paper thickness), i.e.,bulkiness (low density), and ease in turning the pages when the paper isused for books are demanded. In the past, if paper thickness isincreased, stiffness is increased, making it harder to turn the pages.For this reason it was difficult to manage to have both paper thicknessand ease in turning the pages.

[0003] Generally, qualities such as a feel and ease in turning the pagesare the factors that are affected by the suppleness of paper. It is,however, difficult to digitize the suppleness of paper all of a lump,because body, elasticity, strength and other qualities are associatedwith it. With the aim of improving the texture of paper used asbookpaper, in Japanese Patent Laid-open No.1996-246390, thin bookpaperfor which specific spindle-shaped calcium carbonate is used as fillerand in which mechanical pulp with a water-retention value of 100 to 150%is compounded, has been disclosed. In Japanese Patent Laid-openNo.1998-204790, low-density bookpaper of 0.6 to 0.65 g/cm³ in density,which contains 90 weight % or more hardwood kraft pulp (Freeness: CSF500 ml or more) which the hardwood kraft pulp contains 50 to 100 weight% dipterocarp pulp, and which contains calcium carbonate as filler, hasbeen disclosed. As for these bookpapers, however, because it wasnecessary to compound special pulp, they had a disadvantage cost-wise,and they fell short of suppleness and were not superior in texture andease in turning the pages.

[0004] As environmental conservation is gaining momentum, producinglighter paper becomes a subject that cannot be avoided in terms ofeffectively utilizing paper pulp, which is produced from forestresources. Producing lighter paper has become a big current as well fromthe viewpoint of quality requirements for the bookpaper as mentioned inthe above. Here, lighter paper implies reducing the weight of paperwhile maintaining the thickness of the paper, i.e. producing low-density(and bulky) paper.

[0005] As a method of lowering the density (increasing the bulkiness) ofpaper, paper pulp, which is a main raw ingredient of the paper, can befirst examined. For the paper pulp, wood pulp is normally used. As pulpfor reducing the density of paper, because of their stiff fibers,mechanical pulp such as ground pulp, which is obtained by grinding woodby a grinder without using a chemical, or thermo-mechanical pulp, whichis obtained by fibrillating wood by a refiner, is more effective forlowering the density than chemical pulp, which is obtained by a chemicalextraction of lignin, a reinforcing material contained in fibers.Particularly, ground pulp contributes largely to lowering the density.However, ground pulp, which is mechanical pulp, has a problem incompounding it in wood-free paper from the viewpoint of meeting thestandards. Additionally, if it is compounded in the paper, there is aproblem in paper quality (e.g., color reversion) with time.Consequently, compounding the ground pulp is impossible. By the sametoken, it is impossible to compound thermo-mechanical pulp as well.

[0006] In the case of wood-free paper, as for pulp, only chemical pulpcan be compounded. By compounding chemical pulp, paper density issubstantially affected by a pulpified wood type. In other words, therougher and larger wood fibers themselves are, the easier lowering thedensity is possible. For wood-free paper, mainly hardwood pulp iscompounded. Of the hardwood types, gumwood, maple and birch can bementioned as wood types that can be used for lowering paper density. Ina rise of the current environmental conservation trend, however, it isdifficult to collect only these wood types by specifying them forpulping.

[0007] Mechanical paper or wood-containing paper, in which mechanicalpulp is compounded, is a normally lower density paper than wood-freepaper. Compounding stiff fibers causes picking (many of such incidentsare caused by twined fibers derived from mechanical pulp) and lowersstrength. Furthermore, because degree of brightness is degraded byincreasing a compounding ratio of mechanical pulp whose degree ofwhiteness is lower than that of bleached chemical pulp, a compoundingamount of mechanical pulp is restricted. With a recent growing tendencyfor environmental conservation and because of the need for protectingresources, increasing a compounding amount of recycled waste-paper pulpis called on. It is unlikely that recycled waste-paper pulp is pulpifiedby specifically grouping them according to paper quality types such aswood-free paper, newsprint paper, magazine paper, flyer paper, coatedpaper, etc. Instead, all different types of recycled waste paper arepulpified just as mixed. As a result, in terms of pulp qualities,density tends to become higher than that of virgin mechanical pulp. Thereason for this is that fibers contained in recycled waster-paper pulpcomprise a mixture of chemical pulp and mechanical pulp. Because talc,kaolin, clay and calcium carbonate, which are normally used as fillercontained in paper or pigment for a coating layer of coating paper, havea higher density when compared with pulp, by compounding them, thedensity of paper tends to increase. Hence, increasing a compoundingratio of recycled waste-paper pulp tends to increase paper density.

[0008] As mentioned above, if taking the current status of woodresources and quality design of paper into consideration, it is verydifficult to achieve sufficiently low density needed for the paper onlyfrom a pulp aspect.

[0009] Normally, for paper pulp, fibers are made supple by a refiningprocess and are then fibrillated. Because bulkiness tends to decrease bythe refining process, it is desirable not to perform refining process asmuch as possible for the purpose of increasing bulkiness. However, ifthe refining process is insufficient, strength decreases.

[0010] As a method for lowering density when making paper, pressureapplied by a press should be brought down as much as possible duringpress process, and calendaring, which is performed to provide smoothnesson the paper surface, should not be performed. Furthermore, it isdesirable to use as little as possible of a coating amount for surfacecoating of water-soluble polymer such as starch. This coating isperformed to provide the surface strength of paper when being printed.

[0011] In addition to applying some means at the time of pulping andmaking paper, fillers, which are compounded at a higher compoundingratio next to the pulp, have also been examined. For example, a methodfor achieving lower density by compounding hollow synthetic organicmatter capsules as fillers has been disclosed in Japanese PatentLaid-open No.1993-339898. Synthetic organic expandable filler (e.g., aproduct name such as EXPANSEL manufactured by Nihon Filight), whichachieves higher bulkiness by expanding by heat from a dryer portion of apaper machine, has been proposed. The method using these syntheticorganic expandable fillers, however, has such problems that settingdrying conditions is difficult, surface strength is weak and printingglossiness is lowered.

[0012] In Japanese Patent Publication No.1977-39924, a method usingShirasu-balloons has been proposed. In this method, there are suchproblems that they cannot be compounded in paper pulp well, and unevenprinting results occur using the paper made in this method.

[0013] In Japanese Patent Laid-open No.1996-13380, a method for addingmicroscopic fibrillated cellulose has been disclosed. With this method,microscopic fibrillated cellulose has to be fabricated specially. At thetime making paper, it is necessary to adjust freeness of pulp to CSF 400ml or more, preferably to CSF 500 ml or more. It is difficult to adjustfreeness for paper stock in which mechanical pulp is compounded at ahigh compounding ratio; therefore, it is difficult to use this methodfor making mechanical paper and wood-containing paper.

[0014] Using the above-mentioned methods, paper thickness was increased.However, because the paper thickness was increased, the stiffness ofpaper rose exponentially and suppleness of the paper was not improved.As a result, texture, feel and ease in turning the page were notsufficient.

DISCLOSURE OF INVENTION

[0015] The object of the present invention is to provide soft printingpaper, which is satisfactory in its texture, feel and ease in turningthe page and is low in paper density (i.e., high in bulkiness), withwhich paper breaks during printing are less, and which is superior inprintability.

[0016] After devoting themselves to examine to solve the above-mentionedproblems, the inventors of the present invention have found thatprinting paper possessing suppleness, which is satisfactory in itstexture, feel and ease in turning the page and at the same time has highbulkiness, can be obtained by specifying the product of the paperdensity, the breaking length of a paper-making direction and the Young'smodulus of the paper-making direction to be no less than 2×10¹⁸ and nomore than 10×10¹⁸ g·N/m⁴.

[0017] To provide both the suppleness of paper including texture, a feeland ease in turning the page and lightness and bulkiness (tall paperthickness), which are required for bookpaper, the inventors of thepresent invention examined to quantify qualities which paper supplenessaffected. They first examined Clark stiffness and found that Clarkstiffness did not always correlates with actual texture, etc. and thateven with a low Clark stiffness value, satisfactory texture was notalways obtained. Additionally, it was found that the lower the values ofpaper strength and the Young's modulus were, paper texture tended to beexcellent. By increasing paper thickness using conventionally knownmethods for creating bulkiness, suppleness decreased. With this view, byfurther examining paper suppleness, they found that supple paper couldbe manufactured by simultaneously lowering paper strength and theYoung's modulus. In other words, it was found that, to obtain paperpossessing lightness, bulkiness and suppleness, which is the object ofthe present invention, lowering the values of the Young's modulus andthe paper density simultaneously in a balanced manner was effective.After dedicating to further examination, it was found that there wassatisfactory correlation with the product of the paper density, thebreaking length of a paper-making direction and the Young's modulus ofthe paper-making direction. More specifically, it was found that thelower the product of the three was, the suppler and the bulkier (thelower in density) the paper was, and that if the product of the threewas within the limits of 2×10¹⁸ to 10×10¹⁸ g·N/m⁴, the texture and feelof the paper were satisfactory, the paper was light and bulky, and thepaper had less problems in breaks on a paper machine and a printingpress. Particularly, if the product of the three is within the limits of2×10¹⁸ to 5×10¹⁸ g·N/m⁴, the paper concerned is preferable forbookpaper.

[0018] To specify the product of the breaking length of a paper-makingdirection and the Young's modulus of the paper-making direction at theabove-mentioned value, it is necessary that printing paper according tothe present invention be made using a paper machine. In other words, inthe case of paper made using a hand sheet machine, whose fiber directionis nondirectional, it is impossible to adjust the values to the valuesof the breaking length of a paper-making direction and the Young'smodulus of the paper-making direction according to the presentinvention. Even if a hand sheet machine with which an orientation can beprovided is used, density cannot be adjusted to the density according tothe present invention because it is impossible to make the conditionsfor press, drying and calendaring processes uniform with the conditionsfor the paper machine. As mentioned above, by lowering the strength,there are concerns about paper breaks on the paper machine and aprinting press. However, it is inferred that if the Young's modulus islowered simultaneously, paper becomes easy to expand when load isapplied and if it is within the elasticity limits. Consequently, itmakes harder to cause partial concentration of stress, resulting inmaking more difficult to cause paper breaks even if paper strength isdecreased.

[0019] For this reason, as a paper machine, a publicly known machinesuch as a Fourdrinier machine or a twin-wire type machine such as anon-top former type, a hybrid former type or a gap former type machine isused.

[0020] Paper having regular density and the product of the three beingless than 2×10¹⁸ g·N/m⁴ means that its Young's modulus is low. Becausethe paper is exceedingly supple, it does not have body. Besides, becauseits strength is exceedingly low, it becomes easy to break duringpaper-making and printing. Paper with the product of the three beingless than 2×10¹⁸ g·N/m⁴ means that it has excessively low density. Forsuch paper, pressure applied at press and calendaring during thepaper-making process needs to be extremely low. For this reason, itssmoothness is significantly low and it is difficult to use it forprinting.

[0021] In the case of paper having regular density and the product ofthe three being more than 10×10¹⁸ g·N/m⁴, its breaking length isexcessively long or its Young's modulus is high. The paper becomes stiffand its texture lowers. In the case of paper with regular breakinglength and Young's modulus values and the product of the three beingmore than 10×10¹⁸ g·N/m⁴, it means that its density is extremely high,hence it does not serve as the paper according to the present invention,which should be bulky.

[0022] Furthermore, the inventors of the present invention found thatthe breaking length of a paper-making direction was important in papersuppleness. Because the breaking length depends on the strength ofinter-fiber bonding, it is considered that it can be used as an index ofpaper suppleness. If the above-mentioned product of the three values isno less than 2×10^(18,) no more than 10×10¹⁸ g·N/m⁴ and the breakinglength is no more than 4 km, the paper has satisfactory suppleness asbookpaper.

BEST MODES FOR CARRYING THE INVENTION

[0023] In the present invention, confining the product of the paperdensity, the breaking length of a paper-making direction and the Young'smodulus of the paper-making direction within the limits of 2×10¹⁸ to10×10¹⁸ g·N/m⁴ can be achieved by using respective means of lowering thepaper density, the breaking length of a paper-making direction and theYoung's modulus of the paper-making direction independently or incombination. As methods for lowering paper density, increasingcompounding ratios of low-density pulp and low-density filler, use ofbulking chemicals and reducing press pressure during paper-makingprocess can be mentioned. As methods for lowering the breaking length,increasing a compounding ratio of filler and others can be mentioned. Asmethods for lowering the Young's modulus of paper, use of a softeningand others can be mentioned.

[0024] A softening agent used in the present invention should have anaction to block inter-fiber bonding of pulp or to supple fibersthemselves. For example, some surfactants possessing hydrophobic groupsand hydrophilic groups have this action. For example, oil-nonionicsurfactants, sugar alcohol nonionic surfactants, sugar nonionicsurfactants, polyalcohol type nonionic surfactants, higher alcohol,ester compound of polyalcohol and fatty acid, polyoxyalkylene additiveof higher alcohol or higher fatty acid, polyoxyalkylene additive ofhigher fatty acid ester, polyoxyalkylene additive which is an estercompound of polyalcohol and fatty acid, fatty acid polyamideamine, etc.can be mentioned as examples. As long as it can increase papersuppleness, compounds or combinations are not limited to those mentionedabove. Using a surfactant which can lower the breaking length and thedensity in addition to lowering the Young's modulus is one of thepreferred modes for carrying out the present invention.

[0025] To confine the product of the paper density, the breaking lengthof a paper-making direction and the Young's modulus of the paper-makingdirection within the limits of 2×10¹⁸ to 10×10¹⁸ g·N/m⁴, an amount of asoftening agent to be added is determined in consideration of acompounding ratio of pulp, a filler content, internally-added chemicals,etc. Normally, paper should be made by adding a softening agent intopaper stock within the limits of 0.1 to 5 weight % per pulp absolute dryweight.

[0026] As raw ingredient pulp for the printing paper possessingsuppleness according to the present invention, chemical pulp such assoftwood bleached Kraft pulp (NBKP) or unbleached Kraft pulp (NUKP),hardwood bleached Kraft pulp (LBKP) or unbleached hardwood bleachedKraft pulp (LUKP), etc., mechanical pulp such as ground pulp (GP),thermo-mechanical pulp (TMP), chemical-thermo-mechanical pulp (CTMP),etc., deinked pulp (DIP) are used independently or by mixing them at anoptional ratio.

[0027] The pH of the paper possessing suppleness according to thepresent invention can be any of acid, neutral or alkaline. Because thebreaking length and the Young's modulus of paper tend to decrease if thepaper contains filler, containing the filler is preferable. As filler,publicly-known fillers such as hydrated silicic acid, white carbon,talc, kaolin, clay, calcium carbonate, titanium white, synthetic resinfillers, etc. can be used.

[0028] Furthermore, the paper possessing suppleness according to thepresent invention can also contain an alum, a sizing agent, a paperstrength agent, a retention aid, a coloring agent, dyestuff, a deformingagent, etc.

[0029] Additionally, within the limits not affecting the density, thebreaking length and the Young's modulus, a surface-preparation agentmainly comprising water-soluble polymer, etc. can be coated on the paperpossessing suppleness according to the present invention for the purposeof improving surface strength and sizing property (a property to stopblotting).

[0030] As water-soluble polymer, oxidized starch,hydroxyethyl-etherificated starch, oxygen-denaturated starch,polyacrylamide, polyvinyl alcohol, etc., which are normally used as asurface-preparation agent, can be used independently or as a mixture.Additionally, in the surface-preparation agent, in addition to thewater-soluble polymer, a paper durability strengthener, which improveswater resistance and surface strength and an external sizing agentproviding the sizing property, can be added. The surface-preparationagent can be coated using a coating machine such as a two-roll sizepress coater, a gate roll coater, a blade metering coater, or a rodmetering coater etc.

[0031] As mentioned above, by specifying the product of the paperdensity, the breaking length of a paper-making direction and the Young'smodulus of the paper-making direction to be no less than 2×10¹⁸ and nomore than 10×10¹⁸ g·N/m⁴, printing paper that is bulky but light andwhich has excellent suppleness can be obtained. The printing paperpossessing bulkiness and suppleness according to the present inventioncan also be used for offset paper, letterpress printing paper,rotogravure paper, electrophotographic paper or base paper for coatedpaper, inkjet printing paper, thermosensitive paper, pressure sensitivechart paper, etc. as well as for bookpaper.

[0032] The present invention is described below further usingembodiments. The present invention, however, is not limited to thoseembodiments.

Embodiments

[0033] Regarding each paper made as described in the embodiments andcomparative examples below, the density, the breaking length of apaper-making direction and the Young's modulus of the paper-makingdirection of the paper were measured and the product of the three wascalculated. Further, texture of the paper was evaluated.

[0034] Methods used for measuring the above-mentioned items were asfollows:

[0035] Density: In accordance with JIS P 8118-1998.

[0036] Breaking length: In accordance with JIS P 8113-1998, the breakinglength of a paper-making direction was measured and a value obtained wasused as the breaking length of the paper.

[0037] Young's modulus: In accordance with JIS P 8113-1998, the modulusof elasticity in tension was measured and a value obtained was used asthe Young's modulus of the paper.

[0038] Evaluation of suppleness: Texture and a feel were evaluated by 10monitors using four scales of: ∘∘ (Very excellent), ∘ (excellent), Δ(slightly having problems), and × (having problems).

Embodiment 1

[0039] Paper was made by an on-top former type paper machine from paperstock prepared using LBKP (freeness: 350 ml) as pulp and containing 10weight % calcium carbonate per paper weight as filler. Starch was coatedon the paper by an on-machine size press coater with 3.6 g/m² of starchas the coating amount. Thus, wood-free bookpaper was made. Theevaluation results are shown in Table 1.

Embodiment 2

[0040] Paper was made by an on-top former type paper machine from paperstock prepared using LBKP (freeness: 410 ml) as pulp and containing 0.4weight % KB-115 manufactured by Kao Chemicals as a softening agent and28 weight % calcium carbonate per paper weight as filler. Starch wascoated on the paper by an on-machine size press coater with 5.1 g/m² ofstarch as the coating amount. Thus, wood-free bookpaper was made. Theevaluation results are shown in Table 1.

Comparative Example 1

[0041] Paper was made by an on-top former type paper machine from paperstock prepared using LBKP (freeness: 410 ml) as pulp and containing 25weight % calcium carbonate per paper weight as filler. Starch was coatedon the paper by an on-machine size press coater with 3.7 g/m² of starchas the coating amount. Thus, wood-free bookpaper was made. Theevaluation results are shown in Table 1.

Comparative Example 2

[0042] Paper was made by an on-top former type paper machine from paperstock prepared using LBKP (freeness: 345 ml) as pulp and containing 25weight % calcium carbonate per paper weight as filler. Starch was coatedon the paper by an on-machine size press coater with 3.7 g/m² of starchas the coating amount. Thus, wood-free bookpaper was made. Theevaluation results are shown in Table 1.

Comparative Example 3

[0043] Paper was made by a paper machine from paper stock prepared usingLBKP (freeness: 317 ml) as pulp and containing 26 weight % calciumcarbonate per paper weight as filler. Starch and polyvinyl alcohol(Weight ratio: 85:15) were coated on the paper by an on-machine sizepress coater with 4.4 g/m² of starch as the coating amount. Thus,wood-free bookpaper was made. The evaluation results are shown in Table1.

Comparative Example 4

[0044] Paper was made by an on-top former type paper machine from paperstock prepared using mixed pulp (freeness: 350 ml) in which 95 partsLBKP by weight and 5 parts softwood kraft pulp (NBKP) by weight weremixed as pulp and containing 20 weight % calcium carbonate per paperweight as filler. Starch and polyvinyl alcohol (Weight ratio: 85:15)were coated on the paper by an on-machine size press coater with 4.5g/m² of starch as the coating amount. Thus, wood-free bookpaper wasmade. The evaluation results are shown in Table 1.

Comparative Example 5

[0045] Paper was made by a Fourdrinier machine from paper stock preparedusing LBKP (freeness: 350 ml) as pulp and containing 29 weight % calciumcarbonate per paper weight as filler. Starch was coated on the paper byan on-machine size press coater with 3.7 g/m² of starch as the coatingamount. Thus, wood-free bookpaper was made. The evaluation results areshown in Table 1.

Comparative Example 6

[0046] Paper was made by an on-top former type paper machine from paperstock prepared using LBKP (freeness: 360 ml) as pulp and containing 28weight % calcium carbonate per paper weight as filler. Starch was coatedon the paper by an on-machine size press coater with 3.8 g/m² of starchas the coating amount. Thus, wood-free bookpaper was made. Theevaluation results are shown in Table 1.

Comparative Example 7

[0047] Paper was made by an on-top former type paper machine from paperstock prepared using LBKP (freeness: 360 ml) as pulp and containing 28weight % calcium carbonate per paper weight as filler. Starch was coatedon the paper by an on-machine size press coater with 3.8 g/m² of starchas the coating amount. Thus, wood-free bookpaper was made. Theevaluation results are shown in Table 1. TABLE 1 Basis Breaking Young'sDensity × Breaking length × Softening weight Density length modulusYoung's modulus agent (g/m²) (g/cm³) (km) (N/m² × 10⁶) (g · N/m⁴ × 10¹⁸)Texture Added Embodiment 1 79.7 0.57 4.50 3.33 8.54 ◯ No Embodiment 280.7 0.61 2.62 3.14 5.02 ◯◯ Yes Comp. Ex. 1 70.8 0.66 4.95 4.54 14.8 ΔNo Comp. Ex. 2 72.9 0.80 5.29 5.43 23.0 X No Comp. Ex. 3 73.3 0.84 7.506.38 40.2 X No Comp. Ex. 4 78.3 0.81 7.30 6.00 35.5 X No Comp. Ex. 581.8 0.80 5.60 5.81 26.0 X No Comp. Ex. 6 85.5 0.71 4.69 4.45 14.8 Δ NoComp. Ex. 7 88.3 0.67 4.17 4.36 12.2 Δ No

Embodiment 3

[0048] Paper was made by a twin-wire paper machine from paper stockprepared using mixed pulp in which 10 parts NBKP by weight, 35 partsLBKP by weight, 40 parts GP by weight and 15 parts TMP by weight weremixed as pulp and containing 1 weight % KB-115 manufactured by KaoChemicals per pulp as a softening agent and 10 weight % kaolin per paperweight as filler. Starch was coated on the paper by an on-machine sizepress coater with 3.0 g/m² of starch as the coating amount. Thus,wood-containing bookpaper was made. The evaluation results are shown inTable 2.

Embodiment 4

[0049] Paper was made by a twin-wire type paper machine from paper stockprepared using mixed pulp in which 3 parts NBKP by weight, 70 parts GPby weight and 27 parts DIP by weight were mixed as pulp and containing 1weight % KB-08 W manufactured by Kao Chemicals per pulp as a softeningagent. Thus, wood-containing bookpaper was made. The evaluation resultsare shown in Table 2.

Embodiment 5

[0050] Paper was made by a twin-wire paper machine from paper stockprepared using mixed pulp in which 10 parts NBKP by weight, 35 partsLBKP by weight, 40 parts GP by weight and 15 parts TMP by weight weremixed as pulp and containing 1 weight % KB-115 manufactured by KaoChemicals per pulp as a softening agent and 10 weight % kaolin per paperweight as filler. Starch was coated on the paper by an on-machine sizepress coater with 3.0 g/m² of starch as the coating amount. Thus,wood-containing bookpaper was made. The evaluation results are shown inTable 2.

Embodiment 6

[0051] Paper was made by a twin-wire paper machine from paper stockprepared using mixed pulp in which 10 parts NBKP by weight, 35 partsLBKP by weight, 40 parts GP by weight and 15 parts TMP by weight weremixed as pulp and containing 1 weight % KB-115 manufactured by KaoChemicals per pulp as a softening agent and 10 weight % kaolin per paperweight as filler. Starch was coated on the paper by an on-machine sizepress coater with 3.0 g/m² of starch as the coating amount. Thus,wood-containing bookpaper was made. The evaluation results are shown inTable 2.

Embodiment 7

[0052] Paper was made by a twin-wire paper machine from paper stockprepared using mixed pulp in which 9 parts NBKP by weight, 7 parts LBKPby weight, 42 parts GP by weight and 42 parts TMP by weight were mixedas pulp and containing 0.6 weight % KB-115 manufactured by Kao Chemicalsper pulp as a softening agent and 5 weight % calcium carbonate per paperweight as filler. Starch was coated on the paper by an on-machine sizepress coater with 1.8 g/m² of starch as the coating amount. Thus,wood-containing bookpaper was made. The evaluation results are shown inTable 2.

Embodiment 8

[0053] Paper was made by a twin-wire paper machine from paper stockprepared using mixed pulp in which 9 parts NBKP by weight, 7 parts LBKPby weight, 42 parts GP by weight and 42 parts TMP by weight were mixedas pulp and containing 0.8 weight % KB-115 manufactured by Kao Chemicalsper pulp as a softening agent and 5 weight % calcium carbonate per paperweight as filler. Starch was coated on the paper by an on-machine sizepress coater with 1.8 g/m² of starch as the coating amount. Thus,wood-containing bookpaper was made. The evaluation results are shown inTable 2.

Embodiment 9

[0054] Paper was made by a Fourdrinier machine from paper stock preparedusing mixed pulp in which 4 parts NBKP by weight, 40 parts LBKP byweight, 31 parts GP by weight and 33 parts TMP by weight were mixed aspulp and containing 4 weight % amorphous silicate per paper weight asfiller. Starch was coated on the paper by an on-machine size presscoater with 1.9 g/m² of starch as the coating amount. Thus,wood-containing bookpaper was made. The evaluation results are shown inTable 2.

Embodiment 10

[0055] Paper was made by a Fourdrinier machine from paper stock preparedusing mixed pulp in which 9 parts NBKP by weight, 7 parts LBKP byweight, 42 parts GP by weight and 42 parts TMP by weight were mixed aspulp and containing 5 weight % calcium carbonate per paper weight asfiller. Starch was coated on the paper by an on-machine size presscoater with 1.8 g/m² of starch as the coating amount. Thus,wood-containing bookpaper was made. The evaluation results are shown inTable 2.

Embodiment 11

[0056] Paper was made by a Fourdrinier machine from paper stock preparedusing mixed pulp in which 75 parts LBKP by weight and 25 parts TMP byweight were mixed as pulp and containing 0.8 weight % KB-115manufactured by Kao Chemicals per pulp as a softening agent and 20weight % calcium carbonate per paper weight as filler. Starch was coatedon the paper by an on-machine size press coater with 6.0 g/m² of starchas the coating amount. Thus, wood-containing bookpaper was made. Theevaluation results are shown in Table 2.

Comparative Example 8

[0057] Paper was made by a twin-wire paper machine from paper stockprepared using mixed pulp in which 19 parts NBKP by weight, 28 partsLBKP by weight, 20 parts GP by weight, 20 parts TMP by weight and 13parts DIP by weight were mixed as pulp and containing 8 weight % calciumcarbonate per paper weight as filler. Starch was coated on the paper byan on-machine size press coater with 1.8 g/m² of starch as the coatingamount. Thus, wood-containing bookpaper was made. The evaluation resultsare shown in Table 2.

Comparative Example 9

[0058] Regarding wood-containing paper on the market (Product name: NewCream Bulky manufactured by Oji Paper), the evaluation results are shownin Table 2.

Comparative Example 10

[0059] Paper was made by a twin-wire paper machine from paper stockprepared using mixed pulp in which 52 parts NBKP by weight, 8 parts LBKPby weight and 41 parts GP by weight were mixed as pulp and containing 6weight % amorphous silicate per paper weight as filler. Starch wascoated on the paper by an on-machine size press coater with 1.8 g/m² ofstarch as the coating amount. Thus, wood-containing bookpaper was made.The evaluation results are shown in Table 2.

Comparative Example 11

[0060] Paper was made by a Fourdrinier machine from paper stock preparedusing mixed pulp in which 75 parts LBKP by weight and 25 parts TMP byweight were mixed as pulp and containing 20 weight % calcium carbonateper paper weight as filler. Starch was coated on the paper by anon-machine size press coater with 6.0 g/m² of starch as the coatingamount. Thus, wood-containing bookpaper was made. The evaluation resultsare shown in Table 2.

Comparative Example 12

[0061] Paper was made by a twin-wire paper machine from paper stockprepared using mixed pulp in which 6 parts NBKP by weight, 10 parts GPby weight, 16 parts TMP by weight and 68 parts DIP by weight were mixedas pulp. Starch was coated on the paper by an on-machine size presscoater with 0.7 g/m² of starch as the coating amount. Thus,wood-containing bookpaper was made. The evaluation results are shown inTable 2. TABLE 2 Basis Breaking Young's Density × Breaking length ×Softening weight Density length modulus Young's modulus agent (g/m²)(g/cm³) (km) (N/m² × 10⁶) (g · N/m⁴ × 10¹⁸) Texture Added Embodiment 355.2 0.49 3.12 2.04 3.12 ◯◯ Yes Embodiment 4 55.8 0.34 3.53 1.90 2.28 ◯◯Yes Embodiment 5 56.0 0.53 2.97 2.34 3.68 ◯◯ Yes Embodiment 6 65.8 0.553.24 2.34 4.17 ◯◯ Yes Embodiment 7 85.6 0.49 4.14 2.92 5.92 ◯ YesEmbodiment 8 67.0 0.48 3.98 2.70 5.16 ◯◯ Yes Embodiment 9 75.8 0.54 4.603.40 8.45 ◯ No Embodiment 84.8 0.52 4.60 3.18 7.61 ◯ No 10 Embodiment88.4 0.60 4.16 3.55 8.86 ◯ Yes 11 Comp. Ex. 8 58.5 0.77 4.20 4.81 15.6 ΔNo Comp. Ex. 9 61.0 0.57 5.48 3.89 12.2 Δ No Comp. Ex. 10 75.8 0.61 4.503.74 10.3 Δ No Comp. Ex. 11 102.8 0.60 6.28 3.90 14.7 Δ No Comp. Ex. 1245.5 0.62 6.86 5.22 22.0 X No

[0062] As shown in Table 1 and Table 2, if the product of the paperdensity, the breaking length of a paper-making direction and the Young'smodulus of the paper-making direction is within the limits of no lessthan 2×10¹⁸ and no more than 10×10¹⁸ g·N/m⁴, regardless of differencesin pulp composition and filler used, paper is superior in suppleness andserves excellently as bookpaper.

INDUSTRIAL APPLICABILITY

[0063] The suppleness of paper, which indicates qualities such as papertexture, feel and ease in turning the page, complexly relates to body,elasticity, strength and other qualities of the paper, andconventionally it was difficult to digitize these qualities. Aiming atimprovement of a texture as bookpaper, however, the present inventionwas able to digitize the factors affecting the suppleness of the paper.

[0064] Consequently, the present invention was able to providebookpaper, which is light in weight and low in density while itpossesses a texture, a feel, ease in turning the page and bulkiness(tall paper thickness) and at the same time, turning the pages is easywhen the paper is used for books.

1. A soft printing paper which is characterized in that product ofdensity, breaking length of a paper-making direction and Young's modulusof the paper-making direction is no less than 2×10¹⁸ and no more than10×10¹⁸ g·N/m⁴.
 2. The soft printing paper as claimed in claim 1, whichis characterized in that the printing paper is made by a paper machineand that the product of the three is adjusted by using the followingmeans independently or in combination: a means of adjusting paperdensity by improving the compounding ratios of low-density pulp andlow-density filler, using bulking chemicals, or reducing press pressureduring paper-making process, a means of adjusting the paper breakinglength of a paper-making direction by improving the compounding ratiosof low-density pulp and low-density filler, and a means of lowering theYoung's modulus of the paper by using a softening agent.
 3. The softprinting paper as claimed in claim 2, which is characterized in that ituses a component having an action to block inter-fiber bonding of pulpor a component suppling fibers themselves, as the softening agent. 4.The soft printing paper as claimed in claim 2 or claim 3, which ischaracterized in that it uses at least one component selected fromoil-nonionic surfactants, sugar alcohol nonionic surfactants, sugarnonionic surfactants, polyalcohol type nonionic surfactants, higheralcohol, ester compound of polyalcohol and fatty acid, polyoxyalkyleneadditive of higher alcohol or higher fatty acid, polyoxyalkyleneadditive of higher fatty acid ester, polyoxyalkylene additive which isan ester compound of polyalcohol and fatty acid, and fatty acidpolyamideamine, as the softening agent.
 5. The soft printing paper asclaimed in any one of claims 2 to 4, which is characterized in that itis made by a paper machine by adding the softening agent within thelimits of 0.1 to 5 weight % per pulp dry weight to paper stock.
 6. Thesoft printing paper as claimed in any one of claims 1 to 5, which ischaracterized in that a surface-preparation agent mainly comprisingwater-soluble polymer is coated on the base paper of the printing paper.7. The soft printing paper as claimed in claim 6, which is characterizedin that the water-soluble polymer use at least one component selectedfrom starch, oxidized starch, hydroxyethyl-etherificated starch,oxygen-denaturated starch, polyacrylamide, and polyvinyl alcohol.
 8. Thesoft printing paper as claimed in any one of claims 1 to 7, which ischaracterized in that the breaking length of the paper-making directionis no more than 4 Km.